The present invention relates generally to apparatus for deriving time domain measurements of the phase stability of a device being tested, and more particularly to such an apparatus wherein the amplitude of a DC signal indicative of the phase shift introduced by the tested device is compared with the amplitude of a ramp voltage.
Electricaland electronic devices, as well as other devices, introduce phase shift on signals that are coupled through them. The amount of phase shift is not stable within the same device, and there is a drift in phase shift as a function of ambient conditions, such as temperature, power supply voltage, etc. The phase shift instability is frequently referred to as phase noise, and in many types of equipment, it is a parameter that must be or is desirably ascertained. The phase noise can be measured either as a function of time shift (e.g. , as a fraction of a second) or phase shift as measured in degrees or radians. Measuring time shifts has the advantage of providing an absolute indication that is not a function of the frequency of an AC source that is applied to the device being tested, either under test conditions or operating conditions.
Ideally, equipment to measure phase noise should not contribute any noise to the signal being monitored. Practically, this means that the elements of the equipment should introduce negligible phase noise compared to the phase noise of a reference oscillator driving the test device. To insure the introduction of negligible phase noise, it is necessary to measure phase noise with a resolution greater than that available from the best reference oscillators. This implies that one must employ a phase measurement technique which cancels out phase noise of the reference oscillator. In addition, it is desirable to utilize a system employing a minimum number of active elements because of the noise inherently introduced by such elements, particularly on analog signals.
A well-known, simple system which has been employed to measure phase noise, which introduces negligible phase noise compared to the phase noise of a reference oscillator, and is capable of measuring phase noise with a resolution greater than that available from reference oscillators, includes a phase detector having a pair of orthogonal channels that drive a mixer. In one of the channels, the device being tested is connected. The mixter derives a DC output voltage indicative of the phase shift introduced by the tested device, due to drift of the phase characteristics of the device. The DC voltage is applied by an amplifier to a noise analyzer, typically a lock-in amplifier. Because an RF source driving the two orthogonal channels is common to both channels, the phase noise of the RF source does not affect the output voltage of the mixer.
A problem occurs in attempting to apply this simple prior art system to time domain noise analysis. Time domain measures of phase stability, either as a function of time of the phase change with respect to a reference instant or time changes from one cycle to the next, require instantaneous measurement of the phase of a signal at periodic intevals. The instantaneous measurement is usually accomplished by superimposing the phase indicating signal amplitude on a low frequency beat and measuring the interval between adjacent cycles with a period counter and measuring the interval between a reference time and the phase time with a time interval counter.
To produce a beat with the previously described phase detector system, two reference oscillators have been employed. The use of two reference oscillators, however, reintroduces phase noise of the reference oscillators into the output voltage of the mixer so that the advantage of utilizing the phase detector including the mixer is obviated. To overcome this problem, it has been the practice to use a pair of two reference oscillator systems with a dual mixer phase comparator and to employ a time interval counter to measure the difference in the zero crossings of the resulting pair of beats. Although the dual mixer system works quite well, it requires double the circuitry of the simple, single phase detector system, in addition to requiring two reference oscillators, one of which must be offset in frequency from the other.